Plastic products have recently been used for packaging and containers for foods, cosmetics or pharmaceutical products; for horticultural goods such as pots; or for agricultural materials. Used plastic products are either burned or directly dumped as a waste. When the wastes are burned, however, it requires a huge cost and raises environmental problems such as exhaust gas. On the other hand, when the wastes are directly dumped, it also raises an environmental problem of piled-up junks, since those wastes are resistant to degradation in nature. Therefore, biodegradable plastic materials that are easily degraded in nature have been studied and developed in recent years.
Various biodegradable plastic materials are disclosed including a plastic material in which a biodegradable plant material or the like is bound by using a biodegradable binder, a plastic material in which a synthesized biodegradable plastic is used, and a plastic material in which a microorganism or a biodegradable polymeric material yielded by a microorganism is used. With regard to products using a biodegradable plant material, etc., containers made of a biodegradable material is disclosed in, for example, Japanese Laid-Open Patent Application Nos. 2000-229312, 2000-355008, 2001-79816, and 2002-249981, wherein the raw material plant powder containing rice husk as a main component is compressedly molded by hot-pressing by using a binder consisting of a biodegradable substance such as starch, rosin, dammar, copal, gelatin and shellack. Further, Japanese Laid-Open Patent Application No. 2000-229661 describes that a biodegradable container such as a tray, plate, pack and cup is produced from leaves, stems, husks, skins, etc. of reed, straw, corn, bamboo, kaoliang, kenaf, palm and the like by using an edible paste such as a gelatin paste as an adhesive for bonding.
Japanese Laid-Open Patent Application No. 2000-327839 discloses production of a container made of a biodegradable material, wherein the raw material plant powder such as rice husk is mixed with such as bran of cereals like rice bran and molded. Japanese Laid-Open Patent Application No. 2002-23262 discloses a biodegradable plastic, wherein a natural material such as sugarcane, pineapple and seaweeds is refined and used as a main material, which main material is mixed with plant fiber such as corn, pulse, potatoes, old rice, coarse cereal and weed, and the mixture is then molded into the biodegradable plastic by using a binder consisting of a natural material such as persimmon tannin, yam paste powder, pine resin and lacquer. Japanese Laid-Open Patent Application No. 2006-122317 discloses production of a biodegradable sheet, wherein fiber of waste papers is bound by starch paste or yeast slop, or by a biodegradable binder such as polyester resin, polylactide resin, natural rubber resin, and polyvinylalcohol.
Further, Japanese Laid-Open Patent Application No. 7-102114 discloses a biodegradable cellulose ester composition wherein cellulose ester such as cellulose diacetate and cellulose triacetate is mixed with starches and a plasticizer. As for products using a synthetic biodegradable plastic, Japanese Laid-Open Patent Application Nos. 4-335060, 6-340753, 2000-72961, 2001-49098 and 2004-299711 disclose biodegradable resin compositions and compacts using polylactate, copolymer of polylactate, and resin mixture of polylactate.
As for products using a microorganism or a biodegradable polymeric material yielded by a microorganism, products using yeast cells are disclosed, for example, in the following: Japanese Laid-Open Patent Application No. 2002-97301 discloses a biodegradable resin composition consisting of yeast which has been resinified by heating and pressurization; Japanese Laid-Open Patent Application No. 2002-167470 discloses a biodegradable plastic wherein yeast is mixed with a biodegradable plastic such as a resin yielded by a microorganism such as polyhydroxybutyrate, a chemically synthesized resin such as polycaprolactone, polybutylenesuccinate, polyethylenesuccinate, polyglycolic acid, polylactate and polyvinylalcohol, or a denatured resin in which a natural substance such as acetylcellulose and thermoplastic starch are denatured; and Japanese Laid-Open Patent Application No. 7-285108 discloses a wood biodegradable material wherein fiber, which has been dry-fibrillated from waste papers, is mixed with yeast bran followed by pressurization and heating.
Further, as examples of the products using a biodegradable polymeric material yielded by microorganisms, Japanese Laid-Open Patent Application No. 2005-133224 discloses a biodegradable resin molding material to be used as a molding material for such as food containers, wherein pulp fiber such as delignified rice straw, wheat straw, sugarcane chaff (Bargasse), kenaf and Ipil-ipil; β-1,3-D-glucoside polysaccharides yielded by Alcaligenes faecalis, or saccharides such as maltotriose polysaccharides yielded by black yeast (Aureobasidium pullulans) (for improvement of gas barrier property); and a silk fibroin spun by silkworms, or a protein-like substance spun by spiders (for reducing moisture permeability) are mixed. Japanese Laid-Open Patent Application No. 2006-142699 discloses a biodegradable plastic produced by the steps comprising: producing L-lactate by filamentous bacterium Amylomyces rouxii from a raw material containing sucrose such as sugarbeet and sugarcane extracts; and either producing polylactate by dehydrating and condensing the L-lactate, or producing polylactate by first synthesizing cyclic lactide (dimer) from the L-lactate, which cyclic lactide is then subjected to purification followed by ring-opening polymerization.
As for a biodegradable polymeric material yielded by microorganisms, polyester of poly-3-hydroxybutyrate which is a homopolymer of 3-hydroxybutyrate and is yielded by Bacillus megaterium has long been known (M. Lemoigne, Ann. Inst. Pasteur, 39, 144, 1925). For the purpose of improving hard and brittle properties of the poly-3-hydroxybutyrate, Japanese Laid-Open Patent Application Nos. 5-93049 and 7-265065 disclose production of a copolymeric polyester of 3-hydroxybutyrate and 3-hydroxyhexanoate from a fatty acid such as oleic acid or fat such as olive oil by employing a fermentation method using Aeromonas caviae, and disclose use of the copolymeric polyester as a biodegradable material.
As shown above, there has been conducted various studies concerning development of biodegradable polymeric materials, and production methods of biodegradable polymeric materials employing various methods by using various raw materials are disclosed. Conventionally, however, biodegradable polymeric materials having sufficient properties as a polymeric material enough to meet the practical requirements in view of both easy degradability and mechanical strength necessary for use as a film, container, etc., have not been developed. Therefore, such biodegradable materials are currently not practiced sufficiently.    Patent Reference 1: Japanese Laid-Open Patent Application No. 4-335060    Patent Reference 2: Japanese Laid-Open Patent Application No. 5-93049    Patent Reference 3: Japanese Laid-Open Patent Application No. 6-340753    Patent Reference 4: Japanese Laid-Open Patent Application No. 7-102114    Patent Reference 5: Japanese Laid-Open Patent Application No. 7-265065    Patent Reference 6: Japanese Laid-Open Patent Application No. 7-285108    Patent Reference 7: Japanese Laid-Open Patent Application No. 2000-72961    Patent Reference 8: Japanese Laid-Open Patent Application No. 2000-229312    Patent Reference 9: Japanese Laid-Open Patent Application No. 2000-229661    Patent Reference 10: Japanese Laid-Open Patent Application No. 2000-327839    Patent Reference 11: Japanese Laid-Open Patent Application No. 2000-355008    Patent Reference 12: Japanese Laid-Open Patent Application No. 2001-49098    Patent Reference 13: Japanese Laid-Open Patent Application No. 2001-79816    Patent Reference 14: Japanese Laid-Open Patent Application No. 2002-23262    Patent Reference 15: Japanese Laid-Open Patent Application No. 2002-97301    Patent Reference 16: Japanese Laid-Open Patent Application No. 2002-167470    Patent Reference 17: Japanese Laid-Open Patent Application No. 2002-249981    Patent Reference 18: Japanese Laid-Open Patent Application No. 2004-299711    Patent Reference 19: Japanese Laid-Open Patent Application No. 2005-133224    Patent Reference 20: Japanese Laid-Open Patent Application No. 2006-142699    Patent Reference 21: Japanese Laid-Open Patent Application No. 2006-122317    Non-patent Reference 1: M. Lemoigne, Ann. Inst. Pasteur, 39, 144, 1925